MEXICO CITY (AP) — The Mexican government said Tuesday it has temporarily suspended visits by civic, activist and religious groups to migrant detention centers.

Such visits have long served as a safeguard to check on the treatment of migrants, some of whom have complained in the past of crowding, prolonged detention and unsatisfactory conditions.

The National Immigration Institute did not give a reason for suspending the visits, saying only that “rescheduling the visits will depend on the work load of each migrant center, with the goal of providing services to the migrants to continue without interruption.”

The institute said it would notify the groups about when they would be allowed back into the centers, which are generally closed to the press and public.

The move comes just days after hundreds of Central American migrants were detained after attempting to enter Mexico from Guatemala as part of a caravan.

The institute said U.N. human rights and refugee officials had been allowed to visit one of the largest detention centers, located in the southern city of Tapachula, on Friday.